1. Field of the Invention
The present invention relates to television, and more particularly to projection television (TV) systems in which at least one cathode-ray tube (CRT) and an associated lens assembly, together, project a phosphor image of the CRT onto a remote generally planar projection screen to provide a greatly enlarged image.
2. Description of the Prior Art
Present day commercial projection television systems (e.g. rear screen projection TV systems) typically include three different monochrome CRTs which, in combination with their respective lens assemblies, project phosphor images in the three primary colors onto a generally planar projection screen suitably spaced from the lens assemblies. A monochrome CRT includes an electron gun for generating an electron beam and for defining an electron beam axis. The CRT also includes a front portion having an inside screen surface on which an appropriate phosphor screen is located, and also having an outside surface. The inside and outside surfaces are typically planar and are oriented at right angles to the electron beam axis; the electron beam axis passes through a center point of the phosphor screen. During operation of the CRT, the electron beam impinges upon the phosphor screen to produce a primary color phosphor image. The outside surface may include a planar outside surface of a faceplate of the CRT.
The CRT may be liquid cooled in which case the front portion includes a containment plate spaced from the faceplate to form a channel between them, and a transparent cooling liquid filling the channel. The outside surface then includes a planar outside surface of the containment plate. During normal operation of the liquid cooled CRT, the liquid removes unwanted heat from the area of the faceplate. Such liquid cooled tubes are available commercially.
A lens assembly associated with the CRT typically includes a generally flat rear optical surface which is oriented perpendicularly to an optical axis of the lens. The rear optical surface is typically the rear surface of a field flattener element of the assembly. The assembly is located near the CRT outside surface and arranged such that the assembly and the CRT, together, project the phosphor image onto the projection screen.
In commercial rear screen projection TV systems, the three tubes are usually arranged in an "in-line" configuration in which one of the tubes (e.g. green image tube) is located such that its electron beam axis is collinear with its associated optical axis. The optical axis of the lens assembly associated with the green tube passes through a central location of and is normal to a projection surface of the projection screen. The other two tubes (red, blue image tubes) are located off-axis on either side of the normal axis. The electron beam and optical axes of the three tubes lie in a common plane. The respective focusing lenses of the off-axis tubes are skewed (i.e. tilted) so that their respective optical axes converge on the central location of the screen projection surface. These optical axes each make an equal first acute angle (e.g. in a range from 5.degree. to 11.degree.) with respect to the normal axis. The lens assemblies have equal magnifications (e.g. in a range from 7 to 10) and are each spaced an equal optical distance (e.g. in a range from 38" to 53") from the screen projection surface. The electron beam axes of the off-axis tubes also may be collinear with their respective optical axes. However, the focused image planes of the projected red and blue images will not register with the screen projection surface.
In order to register the focused image planes of the phosphor images projected through the off-axis lenses with the screen projection surface, each off-axis tube's phosphor image is rotated relative to the rear optical surface of its associated focusing lens assembly. More specifically, the electron beam axis of the blue image CRT is rotated counterclockwise through a second acute angle and, similarly, the red image CRT is rotated clockwise through essentially the same second acute angle so that each electron beam axis intersects its respective optical axis at the angle of rotation. In other words, for each combination of an off-axis CRT and an off-axis lens assembly, the planes of the CRT outside and the lens rear surfaces intersect at the angle of rotation.
As is well known in the optical art, this angle of rotation (Scheimpflug angle .beta.) is essentially equal to n.theta./M, wherein .theta. is the degree angle between the normal axis of the projection screen and the optical axis of a skewed lens assembly, n is the refractive index of the optical medium disposed between the CRT outside surface and the rear surface of the lens, and M is the numerical magnification of the lens.
Because of fast lenses (e.g. F/1 lens) now often used in projection TV systems, any misregistration of the focused projected-image planes with the screen projection surface is particularly noticeable. It is almost a system design requirement that the Scheimpflug angle be achieved and maintained to within one arc minute in a typical rear screen projection TV system. Therefore, accurately achieving and reliably maintaining the Scheimpflug angle between the CRT and the lens assembly is more critical than in the past.
Prior art arrangements for achieving and maintaining the scheimpflug angle between the CRT and the lens assembly include various holding assemblies such as brackets and the like for orienting the CRT outside surface at the scheimpflug angle with respect to the lens rear surface. A fluid medium (e.g. air, n.perspectiveto.1) is present between the CRT outside surface and the lens rear surface.
These prior art arrangements have drawbacks because they are complicated and expensive to employ, and also because focused image plane misregistration at the screen projection surface is still observed.